The specific aims of this proposal are to determine the cellular factors that define or limit skeletal muscle growth potential and to further elucidate the mechanism by which postnatal undernutrition may cause permanent growth retardation of skeletal muscle. At birth male Sprague-Dawley rats will be assigned to control or restricted groups. The control group will be fed ad libitum throughout the study and the restricted group will receive approximately 50% of the amount of diet consumed by the controls from birth until 90 days using a 50% restriction of the lactating dames during the first 21 days followed by a direct restriction of the pups until 90 days. All litters will be standardized at 8 pups. Throughout the study all animals will receive a purified diet. At 21, 90 210, and 365 days postpartum, 15 animals from each group will be killed and four skeletal muscles from each animal will be analyzed to determine muscle fiber number, diameter, and length as well as muscle nuclear number and protein concentration. These data will be utilized to determine the contribution of each of these morphological and biochemical factors to muscle growth potential and nutritional growth retardation. The long-term objectives of this work are to define the nutritional requirements for protein and energy based on the mass of skeletal muscle and its metabolic activity. These objectives will require examining the effects of specific levels of protein and energy on the cellular development of muscle and the interaction of exercise and age on the maintenance of muscle mass. The applications of this research are to develop a better method for defining nutritional requirements during growth and to define the role of skeletal muscle in regulating whole body metabolism and controlling adult-onset diseases such as obesity and heart disease.